Genetic variation

Genetic Variation in humans (and many other animals) is a product of 3 factors:

1. Arrangement of homologous chromosomes during metaphase:
During metaphase 1 of meiosis(a step in cell division to make sex cells) all the chromosomes that code for the same characteristics (or homologous chromosomes) line up so they can be divided. This lining up is random, and with 23 pairs of homologous chromosomes, there can be 223, or over 8 million different combinations of chromosomes from this alone.

2. Crossing-over
At the beginning of the process to create sex cells the homologous chromosomes get together and parts of the chromosome corss over to swap DNA. This ensures that no matter which chromosome the sex cell recieves it has DNA from both the maternal and paternalgene pool. Each set of chromosomes does this before continuing on.

Each time crossing-over happens, it is different. The average number of swaps made between pairs is 3. Each sex cell will have different genetic information to pass on to the next generation.

3. Random Fertilization
Each egg or sperm, with its unique set of chromosomes, is fertilized randomly. None of the genetic information inside the egg makes IT more likely than the others to mature at ovulation. Sperm (while they do fight, kill eachother and swim at different rates) are random also. None of their abilities is due to the DNA inside them.

Individuals really are unique, and the possibility of anyone else ever having your exact DNA sequence is just about zero. Basically impossible.